Retrofocus-type objective for endoscopes

ABSTRACT

A retrofocus-type objective for endoscopes comprising a front diverging lens group and rear converging lens group, said rear lens group being divided into a first lens group and second lens group, said retrofocus-type objective being arranged to be focused by varying the airspace between said first and second lens groups. By the above arrangement, the front lens is kept fixed and serves also as a cover glass so that the retrofocus-type objective as a whole becomes small in size.

This is a continuation of application Ser. No. 624,823, filed Oct. 22,1975, now abandoned.

BACKGROUND OF THE INVENTION

a. Field of the invention

The present invention relates to an objective for endoscopes and, moreparticularly, to a retrofocus-type objective for endoscopes.

B. Description of the prior art

Known objectives for endoscopes are generally focused by advancing thelens system as a whole. Besides, a retrofocus-type lens system is knownas a lens system by which the image plane can be made flat over a widearea. For the retrofocus-type lens system, however, the aperture stop isarranged in the inside of the lens system. Consequently, the position ofthe entrance pupil comes off the front lens surface of the lens systemand, therefore, the front lens diameter becomes large.

Besides, for endoscopes, a cover glass for waterproof purpose isprovided in front of the objective for endoscopes. When, therefore, theobjective for endoscopes is to be focused by advancing the lens systemas a whole in case of an endoscope employing a retrofocus-type objectivewith a large front lens diameter, the airspace between the cover glassand lens system is to be varied and, consequently, the cover glassdiameter becomes very large. It is, however, not desirable to make thecover glass diameter large because pain of the patient will increase.

SUMMARY OF THE INVENTION

It is, therefore, a primary object of the present invention to provide aretrofocus-type objective for endoscopes comprising a front diverginglens group and rear converging lens group, said rear converging lensgroup being divided into two lens groups, said retrofocus-type objectivefor endoscopes being arranged to be focused by keeping the front lensfixed and varying the total focal length of the rear converging lensgroup by varying the airspace between the said two lens groupsconstituting the rear converging lens group.

When the lens system is focused by keeping the front lens fixed andvarying magnification of the rear converging lens group by varying theairspace between two lens groups constituting the rear converging lensgroup as mentioned in the above, it becomes impossible whenmagnification of the rear lens group becomes close to ×l, to focus thelens system on an object at a distance shorter than the distance at thattime. In this case, the lens system can be focused within the rangewhich satisfies the formula (1) when reference symbol β representsmagnification of the rear converging lens group when the object is atthe infinite distance, reference symbol f_(B) represents the focallength of the rear converging lens group, and reference symbol Δrepresents the amount of movement of the position of the image focusedby the front diverging lens group when the distance to the objectbecomes shorter.

    Δ ≦ f.sub.B (β + (1/β) - 2)         (1)

when the rear converging lens group is divided into two groups asmentioned in the above, i.e., into a first lens group and second lensgroup, relation between magnifications of said first and second lensgroups is expressed the following formula (2) where reference symbol β₁represents magnification of the first lens group when the object is atthe infinite distance, reference symbol β₂ represents magnification ofthe second lens group when the object is also at the infinite distance,reference symbol f represents focal length of the endoscope objective asa whole, and reference symbol f_(F) represents focal length of the frontdiverging lens group.

    β.sub.1 · β.sub.2 = β = f/f.sub.F = constant (2)

From the above formula (2), it becomes

    |β.sub.1 | + |β.sub.2 | ≧ 2√|β|              (3)

That is, the sum of |β₁ | and |β₂ | may be an arbitrary value which islarger than 2√|β|. However, in order to make either |β₁ | or |β₂ |smaller than |β| and at the same time to make it possible to focus thelens system on an object at a short distance, it is necessary to satisfythe following condition.

    |β.sub.1 | + |β.sub.2 | > 1 + β

When the condition |β₁ | + |β₂ | > 1 + |β| is satisfied, either ( β₁ +(1/β₁) ) or (β₂ + (1/β₂) ) becomes larger than (β+ (1/β), and it becomespossible to focus the lens system on an object at a short distance whenit is arranged to focus by moving the lens group for which theabove-mentioned value becomes larger than (β + (1/β) ).

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows a sectional view of Embodiment 1 of the retrofocus-typeobjective for endoscopes according to the present invention; and

FIG. 2 shows a sectional view of Embodiment 2 of the retrofocus-typeobjective for endoscopes according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the retrofocus-type objective for endoscopesaccording to the present invention explained in the above are as shownbelow.

    ______________________________________                                        Embodiment 1                                                                  ______________________________________                                        r.sub.1 =                                                                           6.888                                                                         d.sub.1 = 0.7                                                                              n.sub.1 = 1.51633                                                                          ν.sub.1 = 64.1                             r.sub.2 =                                                                           1.446                                                                         d.sub.2 (Variable)                                                      r.sub.3 =                                                                           -7.022                                                                        d.sub.3 = 1.2                                                                              n.sub.2 = 1.69680                                                                          ν.sub.2 = 55.7                             r.sub.4 =                                                                           -3.565                                                                        d.sub.4 (Variable)                                                      r.sub.5 =                                                                           19.657                                                                        d.sub.5 = 1.2                                                                              n.sub.3 = 1.69680                                                                          ν.sub.3 = 55.7                             r.sub.6 =                                                                           -5.829                                                                        d.sub.6 = 0.2                                                           r.sub.7 =                                                                           7.783                                                                         d.sub.7 = 2  n.sub.4 = 1.69680                                                                          ν.sub.4 = 55.7                             r.sub.8 =                                                                           -2.831                                                                        d.sub.8 = 0.7                                                                              n.sub.5 = 1.78472                                                                          ν.sub.5 = 25.7                             r.sub.9 =                                                                           43.3                                                                    f = 2.493, F.sub.NO = 3.0, 2ω = 70° (Diagonal);                  48°  30' (between opposite sides)                                      f.sub.F = -1.651, f.sub.B = 3.305                                             β = -0.673, β.sub.1 = 6.896, β.sub.2 = -0.098                  ______________________________________                                    

The above Embodiment 1 has lens configuration as shown in FIG. 1 andcomprises a front diverging lens group and rear converging lens group.The front diverging lens group comprises a first lens component L₁ andthe rear coverging lens group comprises a second lens component L₂,third lens component L₃ and fourth lens component L₄. The rearconverging lens group is divided into two groups, i.e., a first lensgroup which comprises the second lens component L₂ and a second lensgroup which comprises the third lens component L₃ and fourth lenscomponent L₄, and the lens system according to Embodiment 1 is arrangedto be focused by varying the airspace d₄ between said first and secondlens groups. For varying said airspace d₄, there are two methods. Onemethod is to move the second lens group and the other is to move thefirst lens group. Numerical values of airspaces d₂ and d₄ when theairspace d₄ is varied by these two methods are as shown below.

    ______________________________________                                        (1) When the second lens group is moved                                                          d.sub.2                                                                              d.sub.4                                             When distance to                                                              object is ∞  2.51   1.5                                                 When distance to                                                              object is 4 mm     2.51   0.43                                                (2) When the first lens group is moved                                                           d.sub.2                                                                              d.sub.4                                             When distance to                                                              object is ∞  2.5    1.7                                                 When distance to                                                              object is 4 mm     4.15   0.05                                                ______________________________________                                    

    ______________________________________                                        Embodiment 2                                                                  ______________________________________                                        r.sub.1 =                                                                           ∞                                                                       d.sub.1 = 0.6                                                                              n.sub.1 = 1.51633                                                                          ν.sub.1 = 64.1                             r.sub.2 =                                                                           1.921                                                                         d.sub.2 = 0.9                                                           r.sub.3 =                                                                           ∞                                                                       d.sub.3 = 6.9                                                                              n.sub.2 = 1.80610                                                                          ν.sub.2 = 40.8 (Prism)                     r.sub.4 =                                                                           ∞                                                                       d.sub.4 (Variable)                                                      r.sub.5 =                                                                           -49.257                                                                       d.sub.5 = 1.0                                                                              n.sub.3 = 1.69680                                                                          ν.sub.3 = 55.7                             r.sub.6 =                                                                           -5.02                                                                         d.sub.6 = 0.2                                                           r.sub.7 =                                                                           4.254                                                                         d.sub.7 = 1.6                                                                              n.sub.4 = 1.62041                                                                          ν.sub.4 = 60.3                             r.sub.8 =                                                                           -3.883                                                                        d.sub.8 = 0.5                                                                              n.sub.5 = 1.84666                                                                          ν.sub.5 = 23.9                             r.sub.9 =                                                                           -7.972                                                                        d.sub.9 (Variable)                                                      r.sub.10 =                                                                          -3.885                                                                        d.sub.10 = 0.5                                                                             n.sub.6 = 1.84666                                                                          ν.sub.6 = 23.9                             r.sub.11 =                                                                          -6.706                                                                  f = 2.178, F.sub.NO = 3.0, 2ω = 85° 28' (Diagonal);              57° 30' (between opposite sides)                                       f.sub.F = -1.935, f.sub.B = 2.703                                             β = 0.825, β.sub.1 = -0.641, β.sub.2 = 1.286                   ______________________________________                                         The above embodiment 2 has lens configuration as shown in FIG. 2 and     comprises a front diverging lens group, prism P and rear converging lens     group. The front diverging lens group comprises a first lens component     L.sub.1 and the rear converging lens group comprises a second lens     component L.sub.2, third lens component L.sub.3 and fourth lens component     L.sub.4. The rear converging lens group is divided into two groups, i.e.,     a first lens group which comprises the second lens component L.sub.2 and     third lens component L.sub.3 and a second lens group which comprises the     fourth lens component L.sub.4, and the lens system according to Embodiment     2 is arranged to be focused by varying the airspace d.sub.9 between said     first and second lens groups. Numerical values of airspaces d.sub.4 and     d.sub.9 when the airspace d.sub.9 is varied are as shown below.

    ______________________________________                                                         d.sub.4                                                                            d.sub.9                                                 ______________________________________                                        When distance to                                                              object is ∞  1.33   1.5                                                 When distance to                                                              object is 5 mm     0.7    2.13                                                ______________________________________                                    

As explained in the above, the retrofocus-type objective for endoscopesaccording to the present invention can be focused without advancing thelens system as a whole, i.e., by keeping the front lens fixed.Therefore, the front lens can be used also as a cover glass forwaterproof purpose. Thus, the present invention provides aretrofocus-type objective for endoscopes which is small in size andwhich can be focused favourably.

I claim:
 1. A retrofocus-type objective for endoscopes comprising afront diverging lens group and a rear converging lens group consistingof a first lens group and a second lens group, said retrofocus-typeobjective for endoscopes being arranged to be focused by varying theairspace between said first and second lens groups while fixing saidfront diverging lens group at a constant position.
 2. A retrofocus-typeobjective for endoscopes according to claim 1 which satisfies thefollowing condition when reference symbol β represents magnification ofsaid rear converging lens group, reference symbol β₁ representsmagnification of said first lens group, and reference symbol β₂represents magnification of said second lens group:

    |β.sub.1 | + | β.sub.2 | > 1 + β


3. A retrofocus-type objective for endoscopes comprising a frontdiverging lens group and rear converging lens group, said frontdiverging lens group comprising a first negative lens component, saidrear converging lens component comprising a second positive lenscomponent, third positive lens component and fourth positive cementeddoublet lens component, said retrofocus-type objective for endoscopesbeing arranged to be focused by integrally moving said third and fourthlens components, said retrofocus-type objective for endoscopes havingnumerical values as given below:

    ______________________________________                                        r.sub.1 =                                                                           6.888                                                                         d.sub.1 = 0.7                                                                              n.sub.1 = 1.51633                                                                          ν.sub.1 = 64.1                             r.sub.2 =                                                                           1.446                                                                         d.sub.2 (Variable)                                                      r.sub.3 =                                                                           -7.022                                                                        d.sub.3 = 1.2                                                                              n.sub.2 = 1.69680                                                                          ν.sub.2 = 55.7                             r.sub.4 =                                                                           -3.565                                                                        d.sub.4 (Variable)                                                      r.sub.5 =                                                                           19.657                                                                        d.sub.5 = 1.2                                                                              n.sub.3 = 1.69680                                                                          ν.sub.3 = 55.7                             r.sub.6 =                                                                           -5.829                                                                        d.sub.6 = 0.2                                                           r.sub.7 =                                                                           7.783                                                                         d.sub.7 =2   n.sub.4 = 1.69680                                                                          ν.sub.4 = 55.7                             r.sub.8 =                                                                           -2.831                                                                        d.sub.8 = 0.7                                                                              n.sub.5 = 1.78472                                                                          ν.sub.5 = 25.7                             r.sub.9 =                                                                           43.3                                                                    f = 2.493, F.sub.NO =3.0, 2ω = 70° (Diagonal);                   48°  30' (between opposite sides)                                      f.sub.F = -1.651, f.sub.B = 3.305                                             β = -0.673, β.sub.1 = 6.896, β.sub.2 = -0.098                  ______________________________________                                    


4. A retrofocus-type objective for endoscopes according to claim 3, inwhich said second lens component in said rear converging lens group isarranged to be moved for focusing instead of said third and fourth lenscomponents.
 5. A retrofocus-type objective for endoscopes comprising afront diverging lens group, a prism and a rear converging lens group,said front diverging lens group comprising a first negative lenscomponent, said rear converging lens group comprising a second positivelens component, third positive cemented doublet lens component andfourth negative lens component, said retrofocus-type objective forendoscopes being arranged to be focused by integrally moving said secondand third lens components, said retrofocus-type objective for endoscopeshaving numerical calues as given below:

    ______________________________________                                        r.sub.1 =                                                                           ∞                                                                       d.sub.1 = 0.6                                                                              n.sub.1 = 1.51633                                                                          ν.sub.1 = 64.1                             r.sub.2 =                                                                           1.921                                                                         d.sub.2 = 0.9                                                           r.sub.3 =                                                                           ∞                                                                       d.sub.3 = 6.9                                                                              n.sub.2 = 1.80610                                                                          ν.sub.2 = 40.8 (Prism)                     r.sub.4 =                                                                           ∞                                                                       d.sub.4 = (Variable)                                                    r.sub.5 =                                                                           -49.257                                                                       d.sub.5 = 1.0                                                                              n.sub.3 = 1.69680                                                                          ν.sub.3 = 55.7                             r.sub.6 =                                                                           -5.02                                                                         d.sub.6 = 0.2                                                           r.sub.7 =                                                                           4.254                                                                         d.sub.7 = 1.6                                                                              n.sub.4 = 1.62041                                                                          ν.sub.4 = 60.3                             r.sub.8 =                                                                           -3.883                                                                        d.sub.8 = 0.5                                                                              n.sub.5 = 1.84666                                                                          ν.sub.5 = 23.9                             r.sub.9 =                                                                           -7.972                                                                        d.sub.9 (Variable)                                                      r.sub.10 =                                                                          -3.885                                                                        d.sub.10 = 0.5                                                                             n.sub.6 = 1.84666                                                                          ν.sub.6 = 23.9                             r.sub.11 =                                                                          -6.706                                                                  f = 2.178, F.sub.NO = 3.0, 2ω = 85° 28' (Diagonal);              57° 30' (between opposite sides)                                       f.sub.F = -1.935, f.sub.B = 2.703                                             β = 0.825, β.sub.1 = -0.641, β.sub.2 = 1.286                   ______________________________________                                    